For me it is not possible to create a vlan tag on top of the lagg0 interface. I am currently investigating /etc/inc/interfaces.inc since there is the function is_vlan_capable, which checks for vlanmtu. I did not change any mtu settings , should I? Both lagg0 and lagg0_vlanXXX show mtu 1500, But the VLAN_MTU flag is present.
The following ifconfig output is from the current running (supermicro c2758) system, which I configured by editing xml-config file myself adding lagg0_vlan1001:
lagg0: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> metric 0 mtu 1500
options=400bb<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,JUMBO_MTU,VLAN_HWCSUM,VLAN_HWTSO>
ether XX:XX:XX:XX:XX:XX
inet6 fe80::XXXX:XXff:feXX:XXXX%lagg0 prefixlen 64 scopeid 0x9
nd6 options=21<PERFORMNUD,AUTO_LINKLOCAL>
media: Ethernet autoselect
status: active
laggproto loadbalance lagghash l2,l3,l4
laggport: igb3 flags=4<ACTIVE>
laggport: igb2 flags=4<ACTIVE>
laggport: igb1 flags=4<ACTIVE>
lagg0_vlan1001: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> metric 0 mtu 1500
options=3<RXCSUM,TXCSUM>
ether XX:XX:XX:XX:XX:XX
inet6 fe80::XXXX:XXXX:XXXX:XXXX%lagg0_vlan1001 prefixlen 64 scopeid 0xa
inet 10.XXX.XXX.5 netmask 0xffff0000 broadcast 10.XXX.255.255
inet 10.XXX.XXX.36 netmask 0xffff0000 broadcast 10.XXX.255.255 vhid 1
nd6 options=21<PERFORMNUD,AUTO_LINKLOCAL>
media: Ethernet autoselect
status: active
vlan: 1001 vlanpcp: 0 parent interface: lagg0
carp: MASTER vhid 1 advbase 2 advskew 0
Sorry if I did something wrong or forgot something. But in the former 2.1 release on my old lanner board it was possible to define the vlans through the web interface. With that it is not possible to do it through the web interface too, the only difference is that there I have three em0 in the lag group instead of the three igb's. I am sorry that I cannot test 2.1 on the supermicro c2758, I think there was the problem with the unsupported igb in freebsd 8.3 in former times.